THE SENSES 907 



for the distance at which a book is usually held), but no power of 

 accommodation remains. The person does indeed contract the 

 pupil in regarding a near object, just as happens in the intact 

 eye ; the most divergent rays are thus cut off and the image 

 made somewhat sharper, and there may appear to be some 

 faculty of accommodation left. But the loss of the whole iris 

 by operation does not affect accommodation in the least ; the 

 iris, therefore, takes no part in it. That no change in the 

 antero-posterior diameter of the eyeball, caused by its deforma- 

 tion by the contraction of the extrinsic muscles, can have any 

 share in accommodation, as has been suggested, is clearly proved 

 by the fact that atropine, which does not affect the action of 

 these muscles, paralyses the mechanism of accommodation. To 

 the consideration of that mechanism we now turn. 



The Mechanism of Accommodation. While everybody is 

 agreed that the main factor in accommodation is the alteration 

 in the curvature of the lens, there is by no means the same 

 unanimity as to the manner in which this is brought about. 

 Helmholtz's explanation, which has long been the most popular, 

 is as follows : In the unaccommodated eye the suspensory 

 ligament and the capsule of the lens are tense and taut, the 

 anterior surface of the lens is flattened by their pressure, and 

 parallel rays (or, what is the same thing, rays from a distant 

 object) are focussed on the retina without any sense of effort. 

 In accommodation for a near object, the meridional or antero- 

 posterior fibres of the ciliary muscle by their contraction pull 

 forward the choroid and relax the suspensory ligament. The 

 elasticity of the lens at once causes it to bulge forwards till it 

 is again checked by the tension of the capsule. 



The explanation of Helmholtz, although widely adopted in the 

 text-books, has not escaped question in the archives. Tscherning 

 has put forward the view that when the ciliary muscle contracts, the 

 suspensory ligament is pulled backwards and outwards. Its tension 

 is thus increased, and the soft external layers of the lens are in 

 consequence moulded upon the harder nucleus, so as to increase the 

 curvature especially around the anterior pole. And Schoen, reviving 

 a similar theory originated fifty years ago by Mannhardt, believes 

 that the ciliary muscle, in contracting, exerts pressure on the 

 anterior portion of the lens, and so increases its curvature. He 

 likens the process to the bulging of an indiarubber ball when it is 

 held in both hands and compressed by the fingers a little behind one 

 of the poles. It will be observed that in both of these theories the 

 suspensory ligament is supposed to be stretched during accommo- 

 dation, not relaxed as Helmholtz supposed. While they have 

 certain advantages over the theory of Helmholtz, particularly in 

 taking account of the presence of radial and circular as well as 

 meridional fibres in the ciliary muscle, they do not agree so well 

 with such experimental tests as have been applied, and therefore 

 Helmholtz's explanation must still be regarded as the best. 



It is supported by the observation of Hess that when the ciliary 



